1. Technical Field
This invention relates generally to methods of charging rechargeable battery cells, and more specifically to a method for terminating the charging cycle associated with a battery cell or cells.
2. Background Art
Rechargeable battery packs are commonly used in portable electronic devices like cellular phones, radios and portable computers. Such battery packs generally contain one or more rechargeable cells in addition to electronic circuitry and mechanical components. Lithium-ion is the chemistry of choice for rechargeable cells in most electronics applications due to its light weight and high energy density.
Lithium batteries must be properly charged to ensure reliable performance. For example, a single lithium cell can generally only be charged until the cell voltage reaches 4.1 or 4.2 volts. Charging the cell beyond this point can result in combustible gasses being generated within the cell, which may compromise operational reliability.
Prior art charging systems generally terminated charge current based upon voltage alone. In other words, these prior art charging systems applied a current to the cells until they reached their termination voltage. Once the termination voltage was reached, the charging system would turn off the charging current.
The problem with these voltage-terminating systems involves ionic relaxation. Briefly, when a current is applied to or pulled from a cell, the active particles that exchange chemical and electrical energy become agitated and bump into each other. When the applied current is removed, the ions begin to return to a state of rest. The resting process is referred to as xe2x80x9cionic relaxationxe2x80x9d. Typically, the time required for relaxation under a normal stimulus is somewhere between 30 and 300 seconds.
Ionic relaxation impacts a cell when charging. When a cell is being charged at a high rate, the voltage across the cell increases as the cell absorbs energy. If the charge current is suddenly interrupted, the cell voltage drops a certain amount almost instantly due to the equivalent series impedance of the cell. Following the initial drop, the cell voltage will continue to drop exponentially until a lower steady state voltage is reached. This exponential decay is a result of ionic relaxation. In a similar fashion, when charge current is applied to the cell, the voltage instantaneously increases due to the equivalent series resistance. This initial jump is followed by an exponential increase in voltage due to ionic agitation.
Charging a battery is similar to, and thus may be visualized as, filling a mug with creamy, frothy root beer. Imagine that the mug is the battery, root beer is energy, and the foamy head is an undesirable increase in cell voltage and impedance caused by inefficient agitation. The goal is to fill the glass with root beer as quickly as possible, i.e. fast xe2x80x9cchargingxe2x80x9d, without any of the foamy head overflowing the mug. Pouring in one continuous stream is the same as charging a battery with a constant current; it generates a substantial amount of head. If, however, one puts in a little root beer and waits for the head to disappear (i.e. allow ionic relaxation to take place), then puts in another burst and so on, the glass can be filled (or battery can be charged) much more quickly.
The problem with voltage-terminating charging systems is that they terminate current when the cell voltage reaches its maximum. Once ionic relaxation occurs, the voltage drops. Thus, while the charger thinks the cell has been fully charged, in reality it may have only been charged to 80% of its capacity. This is analogous to turning off the root beer tap when the frothy head reaches the top of the glass. Once the head subsides, you find that you only have half a glass of root beer.
There is thus a need for an improved charge termination method.